Commonly assigned, published European Patent Application No. 0 675 072 A1 discloses a valve assemblage useful for dispensing liquids from a collapsible container or a rigid, vented container. At the bottom of the container, a first valve is installed in a spout of the container, the first valve including a moveable, spring-biased piston which normally prevents flow of liquid from the container. An associated apparatus, such as a photographic processor into which the liquid is to be dispensed, includes a second valve having a probe member which engages the piston of the first valve to move the piston to a position which permits flow. As the first valve is opened by the probe, the second valve also opens, thus establishing a flow path for liquid from the container into the associated apparatus.
As the associated apparatus operates, the liquid drains from the container. It would be desirable to provide a simple means for detecting and signaling a low liquid level within the container, to enable an operator of the associated apparatus to replace the container in a timely fashion. Various systems are known for detecting liquid level in a vessel, such as those shown in U.S. Pat. Nos. 4,014,010 and 4,714,176. However such systems either require expensive and complex modifications to the vessel to incorporate the detection system, or measure liquid level outside the vessel under circumstances where the vessel may actually have emptied before low level is detected. In the latter case, air may be drawn into the associated apparatus, which could cause operating difficulties.